1. Field of the Invention
The present invention generally relates to electrical machines of the linear or rotary type and, more particularly, to motors powered by direct current and of the reluctance or permanent magnet types.
2. Description of the Prior Art
Electrical motors are currently used to provide motive power in many familiar devices ranging from small actuators to large industrial systems and including transportation vehicles such as urban railroads. In many cases, provision of electrical power to such motors presents no serious problem such as in stationary motor installations or where the power requirements are small and battery power is feasible and relatively economical. In transportation vehicles, however, some difficulties arise due to the mobile nature of the application and the amount of power required. For that reason, the use of electrical motors in vehicles has only been become widespread where power can be supplied through a stationary structure to which the vehicle's motion is constrained, such as in electrically powered railway vehicles.
If the vehicle motion is not so constrained, as in automobiles, power must be provided from batteries which are carried in the vehicle, adding significantly to vehicle weight in an amount often comparable to or greater than the payload of the vehicle. During use of the vehicle, the weight of the vehicle and payload, as well as the batteries, must be accelerated repeatedly, requiring substantial amounts of power simply for the transportation of the batteries themselves. Accordingly, efficiency of the motor becomes of paramount importance in such applications in order to develop an acceptable range and operating time of the vehicle for each recharge of the batteries.
The ability to control operating speed of a motor is also of special importance in many applications, including transportation vehicles. While alternating current motor designs have been able to achieve relatively high efficiencies, motor speed in alternating current machines is largely controlled by the frequency of current used to power the machines and only a limited amount of slip (e.g. in induction motors) is tolerable. Variation of the power supply frequency is often impractical where the power is drawn from commercial electric power distribution systems and, in any event, the apparatus necessary for power frequency control over a wide range is extensive and eliminates much of the efficiency advantages of alternating current machines in applications where variable speed is required.
Direct current machines, on the other hand, can provide speed control by control of input power voltage relative to the load with relatively simple electrical circuitry. In traditional DC motors using commutators, the geometry of the stator and rotor fields are substantially fixed and torque and speed vary with voltage or current applied, the load which must be driven and the windage and other losses in the motor, itself.
Highly precise speed control can be achieved with relatively simple pulse generation circuitry driven from a DC power source and so-called stepping motor types of designs. For example, highly precise speed control is provided in sound recording and reproducing equipment and computer storage devices with so-called pancake motor designs which can be made very compact and of particularly small dimension in the direction of the rotational axis as opposed to prior designs in which a long, multiple pole stator and matching rotors were provided in a generally cylindrical shape and with poles extending generally in the same direction as the rotational axis of the motor.
In designs of stepping motors and in pancake motor designs, in particular, magnetic elements such as permanent magnets or elements of high permeance material are placed at periodically spaced locations on a rotor disk and a sequence of pulses applied to stators located periodically around the rotor and in registration with the path of the magnetic elements in order to attract and/or repel them to cause rotary motion of the rotor. Of course, it should be realized that the same electromagnetic action can be applied to a linear motor since the principle of operation of either type of motor requires only that an electrical pulse be applied when there is some separation between a portion of the stator and a portion of the rotor such that a relative force can be produced between them due to the magnetic field resulting from the pulse.
Consider, for example a simple reluctance machine in the form of a solenoid. Generally the stator will carry a coil to which an electrical current pulse can be applied. The coil will be formed in such a way as to cause a magnetic flux in a magnetic circuit. The magnetic circuit could be defined by no more than air surrounding the coil but is usually defined by a coil core of high permeance material formed in a way to define an air gap in the magnetic circuit through which the rotor can pass. Assuming the rotor to also be a body of relatively high permeance material located at a position where the field of the magnetic circuit will link a high permeance element of the rotor but where reluctance of the magnetic circuit will not be at a minimum, the magnetic field produced by current in the stator coil will cause a force on the rotor in a direction which would reduce reluctance of the magnetic circuit (e.g. if the rotor is permitted to move in the direction of the force, reluctance of the magnetic circuit would be reduced). The same principle applies with permanent magnet motors in which a force occurs in a direction to increase magnetic flux in the magnetic circuit established by the coil core.
The same principle of operation can be applied to stepping motor designs to obtain more-or-less continuous motion. This is done by arranging for pulses to be applied to a portion of the stator when a certain separation occurs between a rotor portion location and the location it would assume for minimum reluctance of the magnetic circuit, removing the current from that stator portion at or before the position of minimum reluctance is reached, allowing the momentum of the rotor portion to carry it beyond the position of minimum reluctance and applying another pulse to another stator section to repeat the process. However, it can be readily understood that such a mode of operation may cause substantial rotational noise since the amount of force generated is proportional to the amount of change of reluctance with rotor position.
For acceptable levels of rotational noise some portion of high permeance material must be present in the air gap of some stator portion when the pulse is applied to the appropriate stator portion so that reluctance changes relatively linearly with position as more of the high permeance material is drawn into the air gap. At the same time, the presence of some high permeance material in the air gap limits the total change in reluctance in the course of interaction of one stator portion or pole with one rotor portion, limiting available torque and power of the motor. On the other hand, a larger excursion of reluctance limits the efficiency of the motor since the magnetic flux in the air gap is limited by the total reluctance of the magnetic circuit, requiring more ampere turns to obtain a high flux density when little or no high permeance material is in the air gap. Then, when high permeance material enters the air gap, saturation of the material defining the magnetic circuit is likely to occur; increasing losses and reducing efficiency.
From the foregoing, it can be readily seen that there are important design trade-offs between rotational noise, power per unit volume of the motor and efficiency of the motor. For this reason, motors and generators following a compact pancake design are generally limited to low power applications and higher power requirements have been answered with the prior cylindrical motor configuration.
Further, known rotor geometries of reluctance and permanent magnet machine rotors and stators of the pancake design type are not dimensionally stable with variations in rotational speed of the motor. For this reason, pancake design motors are generally limited to low and constant speed applications; implying low power although moderate torque can be achieved. It should also be recognized that the number of turns of current-carrying winding on a stator coil increases motor weight and size and can require increase of the length of the magnetic path in the stator and thus increase reluctance thereof as well as increasing copper losses in electrical resistance and saturation and iron losses of the coil cores. Therefore, reduction of current requirements through provision of increased numbers of turns in stator coils is somewhat self-limiting. Additionally, the inductance of coils having high numbers of turns increases inductance and extends the time required to establish and collapse a magnetic field in the coil. This increase of inductance also tends to cause rotational vibration and contributes to loss of efficiency of the motor.